The present invention relates to a wavelength selective optical filter, in particular but not exclusively to a wavelength selective optical filter which is also operable to provide a controllable degree of attenuation.
In contemporary optical communication systems, wavelength division multiplexing (WDM) techniques are used. Such techniques enable many channels bearing communication traffic to be multiplexed onto radiation propagating along a guided optical path, for example an optical fibre. Each channel has associated therewith an allocated range of wavelengths which are used to convey the communication traffic associated with the channel. Thus, WDM techniques allow increased exploitation of optical fibre bandwidth in order to satisfy future demand for enhanced data rates in communication networks.
Use of WDM in contemporary communication systems has created a need for devices which can be connected to optical paths thereof conveying WDM communication traffic, the devices operable to extract communication traffic corresponding to a specific channel without interfering with communication traffic conveyed in other channels; radiation corresponding to these other channels is transmitted through the devices substantially unmodified. Such devices are known as add-drop filters.
There arises a further requirement in contemporary optical communication systems using WDM techniques and incorporating add-drop filters for the filters to be reconfigurable, namely for the filters to be retunable to select different channels. Moreover, it is a yet further requirement that the communication systems should be reconfigurable whilst in operation conveying communication traffic. Thus, each add-drop filter needs to be retunable from a first selected channel to a second selected channel without tuning through channels intermediate between the first and second channels and causing traffic conveyed in these intermediate channels being interrupted or disturbed during retuning. A number of conventional add-drop filters have been reported in the literature and sold commercially which are capable of being tuned from one channel to another. Such conventional add-drop filters incorporate optical filters which tune continuously; as a consequence, they cause disturbance of communication traffic on intermediate channels when being reconfigured. Such optical filters are implemented in a number of ways, for example as cascaded Mach-Zehnder filters fabricated as silicon planar waveguides and as micro-mechanically tuneable Fabry-Perot filters. A U.S. Pat. No. 5,739,945 describes a single cavity continuously-tuneable optical filter incorporating electrostatically actuated mirrors.
Tunable optical filters are known in the prior art.
For example, a U.S. Pat. No. 4,240,696 describes an optical filter including a plurality of adjacent layer pairs, each pair having an incident and an emerging surface. Each pair further comprises a first dielectric layer, a second dielectric layer and a control electrode disposed between and in contact with the layers. The filter additionally includes a plurality of ground electrodes disposed on the layer pairs to electrically contact each incident and emergent surface, a source of electrical potential, and a switch for connecting the source between the control electrodes and the ground electrodes. In the filter, optical radiation is reflected by the filter upon closing the switch and thereby applying the electrical potential in opposite directions across the first and second layers. Electrodes of the layer pairs are thus connected in parallel so that the pairs are not capable of being mutually independently tuned. Moreover, there is no basis in the context of the invention for it to be advantageous to make the pairs independently tunable.
Moreover, in a further example, a U.S. Pat. No. 5,170,290 describes high total transmission tunable comb filter structures. The structures comprise moderately thick layers of optical material having periodic refractive index modulation features comprising a multiplicity of coherently-coupled, weakly-resonant optical cavities. The structures are characterised by spectra of at least order 5 relative to a fundamental lowest-order cavity resonance consisting of narrow, moderate to high density reflection lines occurring in one or more sets, each set being characterised by lines equally spaced by wave number if optical dispersion is neglected. Filters provided by such structures can be electro-optically or mechanically tuned such that the peaks within a spectral band of interest shift by one harmonic order to reflect or transmit optical radiation of any specific wavelength within a band. The cavities are not capable of being mutually independently tuned in the embodiments described in the patent. Moreover, there is no basis in the context of the invention for it to be advantageous to make the cavities independently tunable.
A first approach to providing add-drop filters which do not tune continuously in contemporary systems involves demultiplexing and remultiplexing techniques. Use of such techniques enables add-drop filters to be isolated whilst they are retuned from one channel to another when the systems are being reconfigured. Application of such techniques results in increased insertion loss associated with add-drop filters included within the systems, the insertion loss increasing as the number of channels conveying communication traffic is increased.
A second approach employed in contemporary systems incorporating add-drop filters is for the add-drop filters to include a number of tuneable optical gratings which are tunable from a wavelength intermediate between two neighbouring channels to a given channel. This approach provides a characteristic that filters in the systems are not tuned through a number of channels before reaching their selected channel. However, the approach requires there to be provided a grating for each channel used in the systems, there arising thereby a problem that insertions loss associated with add-drop filters in the systems increases as the number of channels is increased.
There is a further disadvantage that, when the first and second approaches are adopted, add-drop filters are designed for accommodating a specific maximum number of channels; such a maximum number means that the add-drop filters have to be replaced if the number of channels used in the systems are increased by system upgrades to more than the maximum number.
The inventors have appreciated that there is a need for a wavelength selective optical filter capable of incorporation into add-drop filters of communication systems that can tune directly from a first channel to a second channel without tuning through channels intermediate between the first and second channels. Moreover, the inventors have appreciated that the optical filter should be tunable over a relatively large number of channels so that the filters do not need to be replaced when communication system upgrades are implemented.
According to a first aspect of the present invention, there is provided a wavelength selective optical filter for receiving input radiation and outputting corresponding filtered output radiation, characterised in that the filter includes a plurality of mutually independently tunable optical resonators for filtering the input radiation to generate the output radiation, the resonators being at least partially mutually coupled, and the resonators having associated therewith tuning ranges which at least partially mutually overlap.
The filter provides the advantage that it is capable of being tuned from one wavelength to another without tuning through intermediate wavelengths therebetween.
The filter of the invention is distinguished from prior art filters incorporating micro-tuned resonators in that the filter incorporates cavities which are mutually coupled. Such mutual coupling provides a more selective response than merely cascading filters as currently done in the art.
Conveniently, the filter in use is:
(a) at least partially transmissive to the input radiation to generate the output radiation when the optical resonators are mutually tuned to a similar wavelength; and
(b) substantially non-transmissive to the input radiation when the resonators are mutually detuned.
Such a filter provides the desirable characteristic that it is substantially non-transmissive to radiation whilst being retuned from wavelength to another.
Preferably, the coupling from one of the resonators to another resonator adjacent thereto is in a range of 0.01 to 0.1% to obtain a useable degree of selectivity from the filter.
Advantageously, the resonators include first and second tunable Fabry-Perot cavities, the cavities being at least partially mutually coupled through a component common to the cavities. The cavities provide a resonance characteristic when there are an integer number of half wavelengths of radiation propagating between mirrors of the cavities.
Conveniently, the component is a second mirror assembly spatially located between the cavities, the first and second cavities having associated therewith first and third mirror assemblies respectively, the first and second assemblies defining the first cavity, and the second and third assemblies defining the second cavity. The second mirror assembly provides a degree of mutual coupling between the cavities to provide the filter with its wavelength selective response.
When implementing the filter, each cavity preferably includes a void in a region between its associated mirror assemblies. Advantageously, the void is in a range of 10 to 20 xcexcm wide in a direction normal to major planes of its associated mirror assemblies, although 14 xcexcm is its preferred width.
In order to improve response of the filters, the reflectors are preferably distributed Bragg reflectors. Such reflectors can be provided by each assembly comprising multi-layer structures.
AlGaAs, GaAs and AlAs will be used hereafter as abbreviations for aluminium gallium arsenide, gallium arsenide and aluminium arsenide respectively.
Advantageously, each multilayer structure comprises a plurality of alternating layers of AlGaAs and aluminium oxide. Conveniently, the aluminium oxide layers each have a thickness in a range of 300 nm to 350 nm, although 314.5 nm is a preferred thickness.
Moreover, the AlGaAs layers of the first and third assemblies are such that each layer beneficially comprises first, second and third sub-layers so that:
(i) the first and third sub-layers have a composition AlaGabAs where a is in a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42; and
(ii) the second sub-layer has a composition AlcGadAs where c is in a range of 0.28 to 0.32, and d is in a range of 0.68 to 0.72.
Incorporation of the sub-layers enables optical characteristics of the assemblies to be closely controlled in manufacture.
Furthermore, in the third assembly, the first and third sub-layers are preferably each in a range of 10 to 20 nm thick, and the second sub-layer is in a range of 90 to 100 nm thick. Additionally, in the first assembly, the first and third sub-layers are preferably each in a range of 45 to 55 nm thick, and the second sub-layer is preferably in a range of 15 to 25 nm thick. However, 52.8 nm and 19.9 nm are preferred specific thicknesses for these layers.
Advantageously, the second assembly comprises aluminium oxide and AlGaAs layers such that each AlGaAs layer has a composition AlaGabAs where a is in a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42. Preferably, in the second assembly, the AlGaAs layers are in a range of 115 to 140 nm thick. Such layer thicknesses and composition assist to provide a satisfactory assembly reflection characteristic for the filter.
Beneficially, each cavity is tunable by altering a spatial separation between its respective mirror assembly and the second mirror assembly. In order to achieve such tuning, at least part of the mirror assemblies can be resiliently suspended and their mutual spatial separation can be alterable by applying piezo-electric forces to the mirror assemblies. Alternatively, at least part of the mirror assemblies can be resiliently suspended and their mutual spatial separation can be alterable by applying electrostatic forces to the mirror assemblies; the mirror assemblies can be electrically connected to enable potential differences to be applied therebetween to generate the electrostatic forces.
In a practical implementation of the filter, the second and third mirror assemblies each advantageously comprise a central mirror region suspended on a plurality of compliant arms. In one embodiment of the invention, the central mirror region is substantially circular and suspended on four arms. Conveniently, the central region has an effective diameter in a range of 50 to 150 xcexcm and each arm has a length in a range of 600 to 2000 xcexcm, although an effective diameter of 100 xcexcm and a length of 1000 xcexcm are specific preferred dimensions.
For ease of making electrical connections to the filter, the cavities and their associated mirrors can be fabricated onto a substrate in terraced formation to enable electrical connection to be made to the mirror assemblies, the assemblies being mutually electrically isolated.
Advantageously, the filter is fabricated using gallium arsenide or silicon fabrication techniques.
According to a second aspect of the invention, there is provided an add-drop filter for receiving input communication radiation and operable to drop and add radiation corresponding to a specific channel in the input communication radiation, the add-drop filter incorporating a filter according to the first aspect of the invention for isolating radiation corresponding to the channel.
According to a third aspect of the present invention, there is provided a method of fabricating a filter according to the first aspect of the invention, the method comprising the steps of:
(a) forming a series of layers on a substrate, the layers forming mirror assemblies and spacer layers therebetween;
(b) defining features in the layers corresponding to suspended reflectors and associated compliant support arms;
(c) processing the substrate and its associated layers by etching processes to generate the mirror assemblies and optical cavities therebetween where the spacer layers are present;
(d) defining and generating features on the assemblies operable to actuate the mirror assemblies relative to one another; and
(e) mounting the filter produced by steps (a) to (d) above in a carrier and making electrical connection to the filter.
Advantageously, the layers are formed by metal oxide chemical vapour deposition (MOCVD). Moreover, the spacer layers are preferentially wet etched to form the optical cavities and render the mirror assemblies freely suspended.
Conveniently, in the method, the mirror assemblies each comprise alternate layers of AlGaAs and AlAs, the AlAs subsequently heat processed to form aluminium oxide. Such alternate layers provide optimised optical properties for the mirror assemblies.
According to a fourth third aspect of the present invention, there is provided a method of tuning a filter according to the first aspect of the invention from a first wavelength to a second wavelength, the method comprising the steps of:
(a) tuning the resonators to the first wavelength so that the filter provides selective filtration at the first wavelength;
(b) detuning the resonators from the first wavelength by tuning at least one resonator in a first wavelength direction and another resonator in another wavelength direction opposite to the first wavelength direction;
(c) tuning the resonators in a mutually detuned state towards the second wavelength; and
(d) tuning the resonators finally to the second wavelength so that the filter provides selective filtration at the second wavelength.